Conventionally, electrophotography is a method for obtaining a printed image in which a latent charge image is formed on a photosensitive member by various kinds of means using, in general, a photoconductive material; the latent image is developed by a toner and, as needed, the toner image is transferred onto a paper or the like; and the toner image is fixed by heating, pressing, solvent fume, etc., thereby obtaining a printed image.
Methods for developing latent charge images are broadly classified into liquid development methods, which use a developer in which various kinds of pigments or dyes and resins are finely dispersed in an insulating organic liquid, and dry development methods such as a cascade method, a magnetic brush method and a powder cloud method, which use a toner in which a colorant (e.g., carbon black) is dispersed in a natural or synthetic resin. In recent years, dry development methods are widely used from the viewpoint of ease of handling.
In electrophotography, due to its high energy efficiency, a heating roller method is widely and generally used for fixing. However, in such a thermal fixing method, there is such a problem that part of a toner image attaches to the surface of a heating roller upon fixing, transfers onto a copy paper and is likely to cause scumming (that is, a so-called toner offset phenomenon). To prevent the occurrence of the toner offset phenomenon, it is carried out to incorporate a release agent such as wax in a toner.
In the case of incorporating such a release agent in a toner, to prevent the occurrence of the toner offset phenomenon, it is preferable that the release agent is ideally present in the vicinity of the surface of toner particles. It is known that the release agent present in the vicinity of the surface decreases the friction coefficient of toner particles. It is also carried out to improve toner properties by appropriately controlling the friction coefficient.
For example, an image forming method is disclosed in Patent Literature 1, in which an electrostatic image is formed on an OPC photosensitive member having a specific surface hardness; the electrostatic image is developed with a toner to form a toner image, which is such a toner that a specific amount of polyalkylene is contained in a binder resin; the toner image is transferred onto a transferring material; and the toner image on the transferring material is fixed. It is also disclosed that the kinetic friction coefficient of the toner is 0.15 to 0.65. Color toners are disclosed in Patent Literature 2, each of which is composed of at least a binder resin, a colorant and an external additive, and it is also disclosed that there is a specific relationship between the kinetic friction coefficients of the color toners in four colors.
In Patent Literature 3, a toner composed of at least a binder resin and a release agent is disclosed, which is such a toner that the volume average particle diameter and the kinetic friction coefficient of the toner are set in specific ranges. In Patent Literatures 4 and 5, image forming methods using an image forming device having a specific processing speed are disclosed, in which the toner used for development contains a wax (release agent) and the kinetic friction coefficient of the toner is in a specific range.